Armed with our knowledge of the structure of the pressure fields arising from propeller loading and thickness effects (in the absence of blade cavitation) we can seek to determine blade-frequency forces on simple “hulls”. There are pitfalls in so over-simplifying the hull geometry to enable answers to be obtained by “hand-turned” mathematics, giving results which may not be meaningful. Yet the problem which can be “solved” in simple terms has a great seduction, difficult to resist even though the required simplifications are suspected beforehand to be too drastic. One then has to view the results critically and be wary of carrying the implications too far.

From our knowledge that most of the terms in the pressure attenuate rapidly with axial distance fore and aft of the propeller we are tempted to assume that a ship with locally flat, relatively broad stern in way of the propeller may be replaced by a rigid flat plate. The width of the plate is taken equal to that of the local hull and the length extended to infinity fore and aft on the assumption that beyond about two diameters the load density will virtually vanish. As we shall note later, this assumption of fore-aft symmetry of the area is unrealistic as hulls do not extend very much aft of the propeller. We shall also assume at the outset that the submergence of the flat surface above the propeller is large so we might ignore the effect of the water surface.

Cavitation on ship propellers has been the bane of naval architects and ship operators since its first discovery on the propellers of the British destroyer Daring in 1894. Primary interest in propeller-blade cavitation was, for many years, centered upon the attending blade damage and the degradation of thrust arising from extensive, steady cavitation. It was not until the advent of the rapid growth in the size of merchant ships in the past three decades (with concurrent marked increases in blade loading) that extensive, intermittent or unsteady cavitation appeared and was indicted as the cause of large forces exciting highly objectionable hull vibration. Efforts in the modeling of hull wakes in water tunnels date back to about 1955 (cf. van Manen (1957b)) when tests of propeller models in fabricated axially non-uniform flows were being conducted at Maritime Research Institute Netherlands (MARIN), National Physical Laboratory (NPL) and Hamburgische Schiffbau–Versuchsanstalt (HSVA). Non-stationary blade cavities were observed then but there seem to have been no notice or measurement of unsteady near-field pressures attending unsteady cavitation until the experimental work of Takahashi & Ueda (1969). They measured pressures at one point above a propeller in a water tunnel in uniform and non-uniform flow and gave a brief contribution to the 12th International Towing Tank Conference (ITTC) in Rome in 1969. Their principal results are shown in Figure 20.1, where it is seen that the pressure amplitudes increased dramatically with reduced cavitation number.

Here we present the essential steps in the problems posed by the design and analysis of propellers. In design we are required to develop the diameter, pitch, camber and blade section to deliver a required thrust at maximum efficiency (minimum torque). There are other criteria such as to design a propeller to drive a given hull (of known or predicted resistance over a range of speeds) with a specified available shaft horsepower and to determine the ship speed.

The analysis procedure requires prediction of the thrust, torque and efficiency of propellers of specified geometry and inflow.

We begin with the development of the criteria for the radial distribution of thrust-density to achieve maximum efficiency in uniform and non-uniform inflows. This is followed by methods for determining optimum diameter for given solutions and optimum solutions for a given diameter.

The derivation and reason for the induction factors in the lifting-line theory of discrete number of blades, as displayed in the previous chapter, is followed by formulas for the thrust and torque coefficients in terms of the circulation amplitude function Gn.

Applications are then made to the design and analysis of propellers. Means for selection of blade sections to avoid or mitigate cavitation are followed by extensive discussion of practical aspects of tip unloading via camber and pitch variation. Effects of blade form and skew on efficiency and pressure fluctuations at blade frequency (number of blades times revolution per second) are presented.

The pressure fluctuations generated by propellers in the wake of hulls are markedly different from those produced in uniform inflow. The flow in the propeller plane abaft a hull varies spatially as well as temporally. Here we deal only with the effects attending spatial variations peripherally and radially as provided by wake surveys which give the averaged-over-time velocity components as a function of r and γ for a fixed axial location. Temporal variations in the components are aperiodic and cannot be addressed until sufficient measurements have been made to determine their frequency spectra. Ultimately, numerical solutions of the Navier-Stokes equations may provide both spatial and temporal aspects of hull wakes.

Here the spatial variations in the axial and tangential components are reflected in the pressure jump Δp which is taken to vary harmonically with blade position angle γ0. Then we discover a coupling between the harmonics of Δp(γo) and the harmonics of the propagation function yielding a plethora of terms all at integer multiples of blade frequency. Graphical results are given for pressure and velocity fields showing the effect of spatial non-uniformity of the inflow.

We have seen in the previous chapter that the pressure field arising from a lifting-surface model of a propeller in a uniform flow is that due to pressure and velocity dipoles distributed over the blade. Both dipole strengths were constant in time since we considered uniform and stationary inflow.

It is well known that the flow abaft of ships is both spatially and temporally varying. This variability arises from the “prior” or upstream history of the flow produced by the action of viscous stresses and hull-pressure distribution acting on the fluid particles as they pass around the ship from the bow to the stern. Thus the blade sections “see” gust patterns which over long term have mean amplitudes but from instant-to-instant change rapidly with time because of the inherent unsteadiness of the turbulent boundary layer.

Our knowledge of the distribution of flow in the propeller disc is almost entirely based on pitot-tube surveys conducted on big (≈ 6 m) models in large towing tanks and in the absence of the propeller. These are termed nominal wake flows. As is well appreciated, pitot-tube measurements provide only long-term averages of the velocity components at various angular and radial locations in the midplane of the propeller. These measurements depend upon the calibration of the pitot-tube in uniform flow whereas the wake flow radially and tangentially has the effect of shifting the stagnation point on the pitot-tube head, a mechanism not operating in the calibration mode. Thus there is a systematic error which is, to the authors' knowledge, not generally corrected. Moreover, wake-fraction (and thrust-deduction) calculations based upon tests with the same model in several large model basins and upon repetitive tests with the same model in the same large model basin, have shown remarkably different results. A similar scatter was also found in results of wake surveys.

INTRODUCTION

At the introduction of the reinforced plastic products just after World War II the hand lay-up technique was used for the production of those products. This technique has persisted as the years passed on because it is a simple, handcrafted technique with low cost tooling. As with all handcrafted techniques, the quality of the products depends on the individual craftsmanship of the operator. Even with good craftsmanship the products only have one side with defined geometry, the thickness of the products is not well controlled and consequently the fibre-volume fraction can only be guaranteed within a wide range. Moreover the material will contain a considerable number of air bubbles, or voids.

The technique is time consuming and consequently wage expenditure is the greater part of the cost price of the product and the cycle time for moulding is rather long. For volume manufacture of a product several moulds have to be used to achieve sufficient productivity. As the years passed attempts were made both to improve the productivity of the process and to raise the quality of the product.

A first attempt to raise the productivity and to lower labour costs was the spray lay-up technique but this was achieved at the cost of lower mechanical properties. The loss of mechanical properties is due to the fact that with this technique a random fibre distribution in all directions is obtained and tailoring the anisotropy of the material to the loading of the product is not possible.

INTRODUCTION

Design of any artefact is not an activity undertaken in isolation. Although the designer may start, metaphorically, with a blank sheet of paper, he will have a solution in mind based usually on historical or traditional perceptions. There will also be in mind the function or purpose of the artefact which is the reason for the design. Ideally it should be possible to start with the function and to derive the required characteristics of the artefact directly. However, this direction of attack will only be possible for the simplest of objects, and for all practical cases it will be necessary to conceive characteristics and to predict the function from them. If the function does not match that which is required then an iterative procedure is followed.

It is also necessary to be aware of the environment in which the artefact is to function. A description of the environment in terms of loads, temperature, humidity, etc. and the time variation of these will usually be part of the functional specification. If not, it must be derived by the designer before he can start on the design.

Moving on from the philosophy, what is required for the design of plate panels? In the present context a plate panel will be part of a marine structure, and will be loaded by some or all of in-plane biaxial loads, lateral loads and shear, and these loads will frequently be cyclic. The environment is likely to be wet, and in some parts of the structure may well be hot.

INTRODUCTION

The objective of design management is to provide a product design that meets the design brief in an efficient and cost effective manner. It is important that this activity is recognised and respected at all management levels and that the associated organisation is structured such that the activity can be effective. It is an activity that can equally well be practised by a large organisation or an individual, thereby ensuring an effective usage of time and resources leading to a well designed product.

In this instance attention will be given to FRP composite materials and their marine application. Products made in these materials which are likely to form a part of a total product, i.e., the hull of a vessel, the chassis of a ROV, part of an offshore structure, etc., will be discussed. Aspects of design management will therefore be directed at the design activity of these composite products, as opposed to the whole product conception. However, it will be necessary to discuss the role of management at corporate and overall project level, in order to put into perspective the need and role of managing the design activity.

Products involving composite materials require very close management because of the wide variety of choice between materials, processes and structural options. Add to this the complexities of building marine structures, all of which will be affected by weight, cost and the environment, and the importance of careful management increases.

THE NEED FOR MANAGEMENT

Design management is now a recognised subject, but has only recently received the attention it deserves. The driving factor has been better and more competitive products.